1. Field of the Invention
The present invention relates to a developing method for electric latent images as formed in electrophotographic processes, electrostatic recording processes and the like.
2. Description of the Prior Art
In electrophotographic processes, electrostatic recording processes and the like there are known various methods for developing electrostatic images such as a magnetic brush developing method, a cascade developing method and a liquid developing method and the like where a two-component developer mainly composed of a toner and a carrier is employed, and these developing methods give stable images of good quality. However, the developing methods employing a two-component developer suffer from undesirable change of image quality due to the variation of the weight ratio of toner to carrier, deterioration of carriers, scattering of toner particles of dry developers, and dissipation of carrier liquids in liquid developers. In order to eliminate such drawbacks, the developing devices are provided with various means for avoiding such disadvantages, for example, an element for detecting an amount of toners and an agitator for toners and carriers, but this results in an undesirable high cost.
There have been recently proposed one component system developing methods so as to avoid the above mentioned problems. For example, Japanese Patent Publication No. 491/1962 discloses an induction developing method where conductive magnetic toner particles are attached to a sleeve containing a magnet and a magnetic brush composed of toner particles is brought into contact with an electrostatic latent image bearing surface. Since the toner is electrically conductive, an electric charge having a polarity opposite to that of the latent image is induced in each toner particle through a magnet brush facing the electrostatic latent image bearing surface. Thus the development is effected by an electric attraction. Such induction developing method has solved various problems concerning a twocomponent developer system and permits small, light and inexpensive developing devices.
The induction developing method is, however, usually applicable to photosensitive papers having a coating of a photoconductor such as zinc oxide photosensitive papers. Since the toner is conductive, it is very difficult to transfer electrically to plain papers or other image receiving members according to an induction developing method. It has been tried to increase electric resistance of toners in an induction developing method for the purpose of improving the transferring property, but in such case, electric resistance of toner is liable to be affected by the environmental conditions such as humidity and the like and the developed image quality and the transferred images are not stable.
In view of the above mentioned drawbacks of development with one-component developer, U.S. Ser. No. 938,101 filed Aug. 30, 1978, proposes a new developing method where imaged portions only are developed with a magnetic toner without the magnetic toner being brought into contact with the electrostatic latent image bearing surface. According to this new developing method, an insulating magnetic toner comprising a resin and magnetic powder is used, and the toner holds a triboelectric charge mainly caused by a supporting member for developers. Since there is used an insulating magnetic toner holding a stable electric charge, good copies can be obtained even when a copying machine including a transferring step is employed.
U.S. Ser. No. 58,435 filed July 18, 1979 discloses a method for improving reproducibility of gradation, thin letters and small letters by applying an alternating bias electric field to a developing vessel in the above mentioned developing system.